Ab initio study of the reaction mechanism of CO2 with Ti atom in the ground and excited electronic states

Abstract

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Titanium Insertion into CO Bonds in Anionic Ti–CO2 Complexes

“…In the case of titanium–CO 2 interaction, C–O bond breaking occurs readily, and the required energy can easily be supplied in the ion source, even by mere condensation of CO 2 onto the metal atom. These results are reminiscent of the observed bond insertion processes that occur for neutral Ti atoms. ,, Because of the high oxophilicity of titanium, the metal–oxygen bond formed upon bond insertion (forming the metal–oxo ligand) is stronger than the same bond formed for other metals. The calculated Mulliken–Mayer bond orders for the M–O bond in each case are as follows: Ti, 2.11; Fe, 1.52; Ni, 1.29, following the oxophilicity trend of Ti > Fe > Ni .…”

“…As an example of more extreme undercoordination, the interaction of CO 2 with bare, atomic Ti and molecular TiO x ( x = 1–2) has been studied in experimental and computational work on gas-phase and matrix-isolated titanium and titanium oxide neutrals and cations, investigating the structure of CO 2 -single metal atom complexes and the activation that occurs. − Notably, this work showed that insertion of Ti into C–O bonds occurs for neutral Ti and TiO reactions with CO 2 , ,− as well as for Ti + with CO 2 . , In fact, Mascetti and co-workers demonstrated that a Ti atom spontaneously inserts into CO 2 with no barrier, forming a strong TiO bond and a metal carbonyl. , Early transition metals, with their high oxophilicities, are generally likely to form insertion products in interaction with CO 2 , due to the strength of the formed metal–oxygen bond . Other works have shown CO bond insertions in cationic clusters of CO 2 with titanium and vanadium, as well as silicon and nickel. − …”

Computational Exploration of the Direct Reduction of CO2 to CO Mediated by Alkali Metal and Alkaline Earth Metal Chloride Anions

“…The reaction mechanisms relevant for reduction of CO 2 to CO mediated by a single anionic metal center, M – , are illustrated in Figure , and the corresponding computed [MP2/def2-TZVPPD] energies are presented in Table . The fully metal inserted complex OMCO – is reported as a key intermediate for the early transition metals ,,− but turned out not to correspond to a stable minimum energy structure for any of the metal anions studied here. Consequently, we will only need to consider [M,CO 2 ] − intermediates with intact CO 2 cores, consistent with previously published literature on reactions between these metals and CO 2 . − ,, …”

“…It is well established that metal atoms and anions may add CO 2 to form complexes in gas phase reactions, formally metal carbonites (MCO 2 ). Their preferred structures are shown in Scheme , which in the case of alkali and alkaline earth metals is the bidentate coordination of the metal to both oxygen atoms (κ 2 -O 2 C), − while for transition metals, the metal typically binds to the carbon atom (η 1 -CO 2 ) or in a side-on fashion (η 2 -CO 2 ). − Early transition metals, M = Sc, Ti, V, and Cr, ,,− even insert into one of the C–O bonds with subsequent CO elimination: …”

Metal Coordination ofCO2

“…Ti(CO 2 ) complexes in various coordination modes were located on the triplet and quintet potential surfaces; the triplet state (O,O) coordination mode is the most stable one, but it lies above the OTiCO molecule by about 125 kJ mol −1 . Later, Mebel et al 50 made a very detailed study of the same reactions and came to the conclusion that the most energetically favorable reaction mechanism was the insertion of the Ti atom into a C-O bond via an η 2 (C,O)-coordinated Ti(CO 2 ) complex, to produce the triplet OTi(CO) molecule. A comparison of reaction mechanisms for alkaline-earth and early transition metal atoms indicates that, although all of them can enhance CO 2 re-forming into CO, the early transition metal atoms (Sc, Ti, V) are the best ones for this purpose.…”